How to make a menu like the Sculpt Brush/MatCap menu with Python?
The matcap menu is written in C, so you can't define your own menu in Python for showing arbitrary images.

To see where the menu is called...


Open the text editor view.
Right click on the menu, select Edit Source
the line of python script that shows the matcap is displayed.


See that this is a template which means its a preset UI.